1. Field of the Invention
The present invention relates to a boosting circuit and an integrated circuit including the boosting circuit.
2. Description of Related Art
A boosting circuit is built in a driver IC for driving a display apparatus of a mobile terminal. As such a boosting circuit, a charge pump type boosting circuit is known which can output a voltage twice or more as high as an input voltage. In the charge pump type boosting circuit, two or more capacitors may be used. Also, two or more independent power sources may be provided to charge the capacitors. Furthermore, there is a case that a boosted voltage is outputted by connecting the power supply serially to the capacitor.
For example, a charge pump type boosting circuit is known which has two capacitors and can output an output voltage three times as high as an input voltage. In such a case, the charge pump type boosting circuits may operate in response to two input reference voltages. A power supply for supplying the input reference voltages may be provided in the outside of the charge pump type boosting circuit. When two of the input reference voltages supplied from the power supply (hereinafter, to be referred to as an external power supply) are defined as a first reference voltage VR1 and a second reference voltage VR2, it is possible to set the boosted output voltage to a desired value by changing the first reference voltage VR1 and the second reference voltage VR2. When the output voltage of the external power supply is set to an optional value to set the boosted output voltage VOUT to a desired value, it is required to be able to select VR1>VR2 or VR1<VR2, depending of design of components of the boosting circuit. To satisfy the requirement, the boosting circuit is known that can be used in the conditions VR1>VR2 and VR1<VR2, as disclosed in Japanese Patent Application Publication (JP-A-Heisei 8-33321).
FIG. 1 is a circuit diagram showing a configuration of a boosting circuit disclosed in Japanese Patent Application Publication (JP-A-Heisei 8-33321). To simplify the description of an operation of the conventional boosting circuit, the number of capacitors in the circuit is decreased from three to two in a charge pump type boosting circuit 101 shown in FIG. 1. In FIG. 1, VCC and a boosting block described in the conventional circuit are used in common. The charge pump type boosting circuit 101 includes a boosting circuit section 102, a first power supply 103 for supplying the first reference voltage VR1, a second power supply 104 for supplying the second reference voltage VR2 and a rectifier circuit 107. The output voltage VOUT is generated by the boosting circuit section 102 and is outputted from an output node 106.
The boosting circuit section 102 has a first capacitor 111 and a second capacitor 112. The boosting circuit section 102 also has a plurality of switches (first switch 113 to fourth switch 116). The second power supply 104 is provided at the outside of the boosting circuit section 102 and is connected to a first node (negative side) of the first capacitor 111 through the first switch 113 and to a second node (positive side) of the first capacitor 111 through a first diode 117. The second node (positive side) of the first capacitor 11 is connected to the first node (negative side) of the second capacitor 112 through the second switch 114.
The first power supply 103 is provided at the outside of the boosting circuit section 102 and is connected to a second node (positive side) of the second capacitor 112 through a second diode 118. The second node (positive side) of the second capacitor 112 is connected to the output node 106 through a third diode 119 of the rectifier circuit 107. A first node (negative side) of the second capacitor 112 is connected to the first diode 117 through the second switch 114.
The charge pump type boosting circuit 101 performs a boosting operation by switching the plurality of switches (first switch 113 to fourth switch 116). Each of the first switch 113 to fourth switch 116 is activated in response to a clock signal applied thereto. FIG. 2 is a circuit diagram showing a configuration of a clock generating circuit 120 for generating the clock signal used for the boosting operation of the charge pump type boosting circuit 101. A first clock signal 121 is generated from the clock generating circuit 120 and is supplied to the third switch 115 and the fourth switch 116. A second clock 122 is supplied to the first switch 113 and the second switch 114. As shown in FIG. 1, the conventional charge pump type boosting circuit 101 can operate in case of VR1>VR2 and VR1<VR2.
In the conventional charge pump type boosting circuit 101 shown in FIG. 1, a charging voltage of the first capacitor 111 can fall below the second reference voltage VR2 due to a voltage drop by the first diode 117. Similarly, a charging voltage of the second capacitor 112 can fall below the first reference voltage VR1 due to a voltage drop by the second diode 118. The output voltage VOUT of the output node 106 can fall below the voltage appearing at the second node (positive side) of the second capacitor 112 at the time of discharging due to a voltage drop through the third diode 119.
Furthermore, in the conventional charge pump type boosting circuit 101, when the boosting circuit section 102 does not operate, the output voltage VOUT becomes a voltage lower than the first reference voltage VR1 due to a voltage drop by the second diode 118 and a voltage drop by the third diode 119. At this time, given that a voltage difference across the second diode 118 is a first voltage difference VFD2 and a voltage difference across the third diode 119 is a second voltage difference VFD3, when the voltage exceeding (VR1−VFD2−VFD3) is applied to the output node 106 as noise, there is a case that the voltage VOUT of the output node 106 during the non-operation of the boosting circuit section 102 cannot be fixed.